Battery chargers are used in a variety of situations for recharging different types of rechargeable batteries of the wet and dry cell construction. It is desirable to recharge the battery as quickly as possible to ready it for its next use. During high charge rate, the battery temperature may become excessively high which can cause overheating and damage to the battery. Furthermore, the maximum voltage to which the battery is charged should be limited. It is, therefore, important to monitor the temperature and recharged potential of the battery during the charging cycle. In most instances, the temperature of the battery is more critical than the recharge voltage where in a number of instances, temperature indicates that the battery is being overcharged.
A number of systems are available which provide for recharging of batteries and, at the same time, measure voltage and temperature of the battery during the charging cycle.
It is appreciated that several battery charger systems involve the use of four-terminal constructions to provide recharging of the battery and also additional terminals for sensing and detecting battery temperature and voltage. Examples of these systems are found in U.S. Pat. Nos. 3,852,652; 4,065,712 and 4,370,606. U.S. Pat. No. 3,852,652 senses battery temperature through four terminals. The system does not sense the actual battery temperature, but instead detects a rate of change of the temperature. In this way, ambient temperature does not become a factor in determining overheating of the battery during the charging cycle. U.S. Pat. No. 4,370,606 uses a diode system as a battery temperature sensor. The system measures ambient temperature as well as battery temperature. The voltage drop across the diode system, which is changeable as a function of an ambient and battery temperature, is used in establishing battery temperature in the charging process. U.S. Pat. No. 4,065,712 is directed to rapid charging involving the use of intelligent SCR circuitry to reduce current variation with supply voltage or battery voltage. Diodes are used to measure battery temperature. The system switches to a trickle charge upon sensing that the battery is at the desired charge level.
Systems which involve three-terminal constructions for measuring temperature and/or battery voltage are disclosed in U.S. Pat. Nos. 3,531,706; 4,240,022 and 4,394,612. U.S. Pat. No. 3,531,706 discloses a three- or four-terminal device for charging a battery. The battery temperature is sensed in conjunction with battery potential and the charging cycle is switched to trickle charge when predetermined levels of temperature and/or voltage are attained. A thermal switch is used in the three terminal charging device of U.S. Pat. No. 4,240,022 to cut off the high rate of charge when the battery reaches full charge. The battery is then trickle charged for the remaining period of charging. A comparator system is used in U.S. Pat. No. 4,394,612 to determine the battery voltage when charging current is not flowing. The system requires a three-terminal charging connection which relies on current flowing out of the battery when charging current is not flowing to permit the comparator system to determine if the battery is at full charge.
Examples of two-terminal charging systems are disclosed in U.S. Pat. Nos. 3,626,270 and 3,911,351. The U.S. Pat. No. 3,626,270 is directed to the rapid charging of nickel cadmium batteries. The battery is subject to alternating charging and discharging intervals. Either battery voltage or temperature may be monitored to determine when rapid charging should cease with conversion to trickle charge. Temperature measurement, according to this disclosure, requires the use of at least one additional terminal for the temperature sensor if the battery and the charging device are not to be within chose physical proximity. U.S. Pat. No. 3,911,351 includes switches and latches which switch the high charge rate for the system to a trickle charge on the battery when battery voltage or temperature exceeds predetermined upper limits. A thermal switch is used to sense battery temperature which is placed in parallel with a resistor and a capacitor. A first SCR is used to control charging of the battery while a second SCR will turn off the charging when a predetermined charge point is reached. The peak voltage across the battery during charge is measured, which results in variation in charging current. This will affect the actual battery voltage at shut-off. The circuit is designed to take a higher battery voltage when a preset temperature is reached. The charger then responds to this new voltage as though the battery were fully charged. The system is subject to errors caused by variation in charging current due to battery condition and main supply of voltage and also due to variation in battery impedance. These two-terminal systems for charging batteries do not provide for an accurate measurement of battery voltage and/or temperature during the charging cycle with resultant errors in the charging cycle which can either damage the charged battery or provide an incompletely charged battery.